During downhole operations, in which tubing is concentrically located within casing or the borehole, there are two flow directions. In one flow direction, referred to as normal circulation, fluid flows down the tubing and up the annulus between the tubing and casing. In the other flow direction, referred to as reverse circulation, fluid flows down the annulus between the tubing and casing and up the tubing.
Sometimes during downhole operations, as for example during fraccing, it is necessary to allow both normal and reverse circulation, but when hydrocarbons, foreign fluid or gases are present downhole, it is preferable only to permit normal circulation.
In the present state of the art this is accomplished by pulling the downhole tools used for two way circulation out of the well and reentering the well with one way circulating tools.